Long Term Evolution, LTE, is a standard for high-performance wireless communication developed by the 3rd Generation Partnership Project, 3GPP, with an aim to improve the network capacity, simplify the network architecture, provide improved mobility support, improve the spectrum flexibility, increase the data rates, and reduce latency among other things.
One of the basic principles of the LTE radio access procedure is shared-channel transmission, where radio transmission resources such as time-frequency radio resources are dynamically shared between users. These resources can be shared by dynamic scheduling where the network, usually the eNodeB, takes scheduling decisions and sends scheduling grants to requesting user terminals [1].
However, there is also a possibility in LTE to reduce the control signaling by so-called persistent or semi-persistent scheduling when resource allocation is done on a periodic basis and the scheduling decision applies to every nth subframe [2].
The network can also assign a set of radio resources for contention between users; so-called contention-based transmission [3-6]. In particular, for uplink transmission, contention-based access allows a UE to transmit data without having to first transmit a scheduling request and wait for the corresponding grant. Although contention-based transmission can be seen as a form of unscheduled access or direct access, the overall set of radio resources available for contention between the users is still normally allocated by the network.
Latency is an important characteristic in wireless communication, and it is generally desirable to reduce the latency experienced by wireless devices such as User Equipment, UEs, in wireless communication networks. This holds true for ordinary cellular communication networks such as that illustrated in FIG. 1, in which a network node such as a base station serves a number of UEs, but is equally important for device-to-device communication.
By way of example, contention-based transmission has been proposed as a means to reduce latency for transmissions in LTE. This is sometimes also referred to as direct access. Recently, contention-based transmission has also been brought up as a candidate to reduce control signaling overhead for small data transmissions.
In the case of uplink traffic, it is most beneficial for UEs to remain in Radio Resource Control, RRC, Connected Mode to avoid the delays and hand-shaking associated with RRC Connection Setup. If extended Discontinuous Reception, DRX, cycle lengths are introduced in RRC Connected mode, UEs could further remain there for very long periods of time to provide low latency and low control signaling overhead without compromising battery life. With a contention-based approach, UEs could avoid having to send a scheduling requests and wait for the replies and instead instantly transmit their data; however at the risk of collision.
FIG. 2 is a schematic diagram illustrating an example of prior art transmission based on grant allocation according to legacy operation when the UE has synchronization.
When a UE has uplink synchronization, i.e. the Timing Advance, TA, timer has not expired, a UE will in legacy operation simply send a Scheduling Request, SR with a Buffer Status Report, BSR, to receive a suitable grant for the transmission of its data.
FIG. 3 is a schematic diagram illustrating an example of prior art transmission based on grant allocation according to legacy operation when the UE is initially out-of-synchronization.
In RRC Connected Mode when the UE does not have uplink synchronization, i.e. the TA timer has expired, UEs will have to do a Random Access, RA, to get the TA in the RA reply. Once the UE is synchronized, it can send the SR request to receive a grant for transmission of data.
The RA reply in legacy operation also contains a minimal grant which the UE can use to transmit the SR request and the BSR. This grant however is minimal to provide a very robust RA solution. This means that it is unlikely that any user plane data would fit in this first small grant.
FIG. 4 is a schematic diagram illustrating an example of prior art transmission based on contention-based, CB, access to the transmission resources. The eNB sends an initial grant indicating the set of radio resources on which the UE is allowed to transmit. The grant sent from the eNB may also include an indication of the modulation and coding scheme to be used for data transmission. This so-called CB grant is typically transmitted with a robust transport format and Transport Block Size, TBS. The UE can then contend with other UEs for direct access to the radio resources allocated for uplink transmission by the CB grant. Usually, the CB grant points to a particular uplink transmission resource so that the grant is transmitted from the eNB for every uplink data transmission occasion.
A contention-based solution might be favorable, since at least one round-trip could be skipped by transmitting the data directly, without having to wait for a grant (FIG. 2) or without having to first wait for random access round-trip and then wait for the grant (FIG. 3).
With the use of extended DRX cycles it would also become more plausible that UEs would be out-of-synchronization and therefore experience both problems in legacy operation (FIG. 3).
However, in contention-based access, the UE buffer status is generally not known by the network and consequently it would not be possible to optimize the TBS. A small and robust TBS would rather have to be used for all UEs in order to serve the cell-edge UE with the worst radio conditions in a cell. Because of this, conventional contention-based transmission (FIG. 4) may likely be worse than legacy operation (FIG. 2 or FIG. 3) since having to transmit little data in many consecutive subframes may be worse than waiting for a suitable legacy grant to send it all at once.
Conventional contention-based access thus have clearly conflicting properties in that the latency and control signaling overhead is reduced, while the small and robust TBS required to ensure reliable transmissions will lead to a situation where a relatively large number of small data transmissions are required to transfer a given amount of data.
Similar problems with latency, control signaling overhead and data transfer rates can also be encountered in scenarios based on persistent and/or semi-persistent scheduling.